The paper presents a new method for predicting the frequency of the b(1) mode, which is infrared-inactive, in complexes of the type LM(CO)(5) belonging to C-4V point group. The method was based on the relation lambda(3) = lambda(4) + [(1 - delta/delta)](lambda(1) - lambda(2)), where delta = (lambda(1) - lambda(2))/(lambda(1) - lambda(2) + lambda(3) - lambda(4)), lambda(1), lambda(2), lambda(3) and lambda(4) are the lambda parameters of the a(1)((1)), a(1)((2)), b(1) and e modes, respectively. For a large numbers of complexes of the type LM(CO)(5) the average value of delta was found to be 0.80, with a standard deviation of 0.02. With the use of average value of 6, the frequencies of b(1), mode were estimated. The result obtained indicated that there exists a rather good fit between observed and calculated frequencies, with a mean error of 2.7 cm(-1). In addition, it was shown that the 6deltaparameter can be used as a criterion of the correct band assignment for the complexes understudy. (C) 2004 Elsevier B.V. All rights reserved.